Syndiotacticity and isotacticity are stereospecific structural relationships which may be involved in the formation of stereoregular polymers from various monomers. Stereospecific propagation may be applied in the polymerization of ethylenically unsaturated monomers such as C.sub.3 + alpha olefins, 1-dienes such as 1,3-butadiene, substituted vinyl compounds such as vinyl aromatics, e.g., styrene or vinyl chloride, vinyl chloride, vinyl ethers such as alkyl vinyl ethers, e.g., isobutyl vinyl ether, or even aryl vinyl ethers. Stereospecific polymer propagation is probably of most significance in the production of polypropylene of isotactic or syndiotactic structure.
Syndotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the main polymer chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Pat. No. 3,258,455. As disclosed in this patent, syndiotactic polypropylene can be produced by using a catalyst prepared from titanium trichloride and diethylaluminum monochloride. A later patent to Natta et al., U.S. Pat. No. 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. U.S. Pat. No. 3,364,190 to Emrick, discloses the use of a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a phosphorus-containing Lewis base in the production of syndiotactic polypropylene.
As disclosed in the aforementioned patents, and as is known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or below the plane. Using the Fischer projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: ##STR1##
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmm . . . with each "m" representing a "meso" dyad, or successive methyl groups on the same side of the plane. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer. In contrast to the isotactic structure, syndiotactic propylene polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Syndiotactic polypropylene is shown in zig-zag representation as follows: ##STR2##
Corresponding representations for syndiotactic polyvinylchloride and polystyrene, respectively, are: ##STR3##
Using the Fischer projection formula, the structure of a syndiotactic polymer or polymer block for polypropylene is designated as: ##STR4##
In NMR nonmenclature, this pentad is described as . . . rrrr . . . , in which each "r" represents a "racemic" dyad, i.e., successive methyl groups on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene. This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer, which is soluble in xylene. An atactic polymer exhibits no regular order of repeating unit configuration in the polymer chain and forms essentially a waxy product.
While it is possible for a catalyst to produce all three types of polymers, it is desirable for a catalyst to produce predominantly isotactic or syndiotactic polymers with very little atactic polymer. Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. No. 4,794,096, issued Dec. 27, 1988 and U.S. Pat. No. 4,975,403, issued Dec. 4, 1990. These patents disclose chiral, stereorigid metallocene catalysts that polymerize olefins to form isotactic polymers and are especially useful in the polymerization of highly isotactic polypropylene.
As disclosed, for example, in the aforementioned U.S. Pat. No. 4,794,096, stereorigidity in a metallocene ligand is imparted by means of a structural bridge extending between cyclopentadienyl groups. Specifically disclosed in this patent are stereoregular hafnium metallocenes, which may be characterized by the following formula: EQU R"(C.sub.5 (R').sub.4).sub.2 HfQ.sub.p ( 1)
In formula (1), (C.sub.5 (R').sub.4) is a cyclopentadienyl or substituted cyclopentadienyl group, R' is independently hydrogen or a hydrocarbyl radical having 1-20 carbon atoms, and R" is a structural bridge extending between the cyclopentadienyl rings. Q is a halogen or a hydrocarbon radical, such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl, having 1-20 carbon atoms and 0&lt;p&lt;3.
Catalysts that produce syndiotactic polypropylene or other syndiotactic polyolefins and methods for the preparation of such catalysts are disclosed in the aforementioned U.S. Pat. No. 4,892,851. These catalysts are also bridged stereorigid metallocene catalysts, but, in this case, the catalysts have a structural bridge extending between dissimilar cyclopentadienyl groups, and may be characterized by the formula: EQU R"(C.sub.p R.sub.n)(C.sub.p R'.sub.m)MeQ.sub.k ( 2)
In formula (2), C.sub.p represents a cyclopentadienyl or substituted cyclopentadienyl ring, and R and R' represent hydrocarbyl radicals having 1-20 carbon atoms. R" is a structural bridge between the rings imparting stereorigidity to the catalyst. Me represents a transition metal, and Q a hydrocarbyl radical or halogen. R'm is selected so that (C.sub.p R'.sub.m) is a sterically different substituted cyclopentadienyl ring than (C.sub.p R.sub.n); n varies from 0 to 4 (0 designating no hydrocarbyl groups, i.e., an unsubstituted cyclopentadienyl ring), m varies from 1-4, and K is from 0-3. The sterically different cyclopentadienyl rings produce a predominantly syndiotactic polymer rather than an isotactic polymer.
Specifically disclosed in U.S. Pat. No. 4,892,851, are synthesis procedures for the preparation of bridged metallocene ligands having a dissimilar cyclopentadienyl group by the reaction of 6, 6 dimethyl fulvene with a substituted cyclopentadiene, such as fluorene, to produce a ligand characterized by an isopropylidene bridge structure. The ligand coupling reaction is carried out under relatively cold temperature conditions and is characterized by the addition of a relatively concentrated solution of the substituted fulvene to a relatively dilute solution of the fluorene, both in a polar solvent, specifically, tetrahydrofuran. After formation of the ligand, aromatization may be accomplished by the addition of n-butyl lithium.
The aforementioned parent application Ser. No. 418,886, discloses stereorigid cationic metallocenes, including, inter alia, bridged metallocene catalysts useful for the production of syndiotactic polymers. The bridged metallocene catalysts of application Ser. No. 418,886 comprise an unbalanced metallocene cation and a stable, non-coordinating counteranion for the metallocene cation. The metallocene cation is characterized by a cationic metallocene ligand having sterically dissimilar ring structures joined to a positively charged coordinating transition metal atom. The dissimilar cyclopentadienyl rings, at least one of which is substituted, are both in a stereorigid relationship relative to the coordinating metallocene of the metal atom catalyst, and, as noted previously, the stereorigid relationship may be imparted by means of a structural bridge between the dissimilar cyclopentadienyl rings.
Cationic metallocene catalysts of yet another type are disclosed in EPO Pat. Nos. 277,003 and 277,004 to Turner. As disclosed in these patents, a bis(cyclopentadienyl) zirconium, titanium, or hafnium compound is reacted with a second compound comprising a cation capable of donating a proton or an ion exchange compound comprising a cation which will irreversibly react with a ligand on the first compound, and a bulky, stable ion. The catalysts described in the European Patents Nos. 277,003 and 277,004 are disclosed as especially useful in the polymerization of ethylene, and more generally, in the polymerization of alpha olefins, diolefins and/or an acetylenically unsaturated compound containing from 2-18 carbon atoms. Stereospecificity, or lack thereof, is not generally discussed in these patents, although in 277,004, examples are given of producing atactic polypropylene, and in one instance (Example 39), isotactic polypropylene.